1. Field of the Invention
This invention relates to an ink-jet recording pigment ink which has superior dispersion stability and storage stability, and can achieve a high ejection stability even when used in an ink-jet recording process of a system in which heat energy is used as ejection energy to eject inks. More particularly, this invention relates to an ink-jet recording pigment ink which, where images are printed on gloss paper used generally as recording mediums for ink-jet recording, can give images having a sufficient scratch resistance and also cannot easily cause offset or the like also when some sheets of gloss paper are superposed.
2. Description of the Related Art
As inks used in ink-jet recording, which are used in ink-jet recording processes, dye inks have been prevalent which make use of, as colorants, dyes which are soluble in the chief-component water. The dye inks have advantages that they have superior long-term storage stability and can not easily cause any nozzle clogging during ejection and also that print with superior chroma and transparency can be obtained. However, they, on the other hand, have a problem that the print obtained lacks in weatherability and water resistance.
Accordingly, in recent years, in order to improve weatherability of the print, pigment inks making use of pigments as colorants attract notice and have come to be in use. The pigments are not soluble in water, and hence the pigments must be dispersed in ink solvents in order for the pigment inks to be used as inks. It, however, is difficult to keep a good state of dispersion of pigments over a long period of time while keeping pigment particles from agglomerating or settling. Thus, no stable ejection performance is achievable in some cases. Especially where an electrothermal transducer is used as a means for supplying the ejection energy, air bubbles are generated by providing an ink with heat energy, to eject ink droplets, and hence it is expected that physical properties of the ink change abruptly because of the heat energy at the time of bubbling. Accordingly, where pigment inks having a low dispersion stability are ejected by the ink-jet recording system, it has sometimes come about that no stable ejection performance is achievable because of, e.g., deposits having formed in nozzles.
Meanwhile, in the case when the pigment inks are used, problems as stated below may arise with regard to ink-jet recording mediums as well. In order to make the recording performable in a high image quality, an ink-jet recording medium has conventionally been used in which an ink-receiving layer having voids in a large number is provided so as for the recording medium to be improved in ink absorptivity. Where images are formed on such an ink-jet recording medium, in the case of the dye ink, the ink is immediately absorbed in the ink-receiving layer because dye molecules stand dissolved in the ink, so that recorded images with a high image quality can be obtained. However, in the case when images are printed on such a recording medium by using the pigment ink, most pigment particles standing dispersed in the ink are larger than the voids of the ink-receiving layer of the recording medium, and hence these pigment particles come to be fixed to the recording medium surface. As a result, recorded images formed of the pigment ink have a scratch resistance inferior to the case when the dye ink is used. There has been such a problem. Further, where images are continuously printed on a plurality of such recording mediums, another problem has sometimes come about such that, when recorded images are put upon one another through recording mediums on a paper delivery tray, the recorded images set off to the back of the recording mediums superposed to come to stick together. The present inventors have considered whether this problem could be resolved by improving the releasability between the recorded images and the recording medium, where they made studies on the background art.
As a technique by which releasability can be improved, it has hitherto been common to use silicone materials in various fields. For example, as products making use of sheets improved in releasability, they may include release sheets used for various kinds of functional sheets and films having pressure-sensitive adhesive force or other adhesive force. The release sheets are used to protect pressure-sensitive adhesive layers or other adhesive layers of various kinds of functional sheets and films having pressure-sensitive adhesive force or other adhesive force. More specifically, these are used in order to prevent the pressure-sensitive adhesive layers or other adhesive layers from coming into contact with foreign materials to stick together or prevent any dirt from adhering to lower the pressure-sensitive adhesive force or other adhesive force, by the time that these various kinds of functional sheets and films are stuck to any adherends. Accordingly, in order to protect the pressure-sensitive adhesive force or other adhesive force of functional sheets and films, such release sheets must be kept to adhere firmly to the films or the like. On the other hand, the release sheets must be peeled from the functional films or the like when the films or the like are stuck to adherends, thus the releasability is an important factor. This releasability is commonly obtained by coating a base material surface with a silicone type material, and it is common to use the silicone type material to impart such releasability.
Here, the silicone type material refers to a material having structure wherein siloxane linkages formed of i) silicon atoms to which alkyl groups or the like stand bonded and ii) oxygen atoms are arranged in a large number. Characteristic features of such a silicone type material are that, because of its strong linkage between silicon and oxygen, it has high thermal and chemical stabilities and also has high electrical insulating properties. Also, because of its small mutual action between molecules and low interface tension (surface tension), it is variously applied in various fields as a material having good releasability or release properties and also having a high water repellency. Taking note of the use of such a silicone type material in inks for ink-jet recording, the following two types of inks are conventionally known in the art. That is, they are an ink to which a silicone type surface-active agent has been added and an ink to which a silicone type polymer has been added.
Of the foregoing, there are many proposals in which the silicone type surface-active agent has been added. All of such proposals, however, are made in order to change the surface tension that is a liquid physical property of inks. Hence, in all cases, the silicone type surface-active agent is limited to its use as a permeating agent for improving permeability of ink into paper or as an antifoaming agent which prevents ink from foaming (see Japanese Patent Applications Laid-open No. H08-193177 and No. 2003-105236). To add such a surface-active agent in a large quantity means to extremely lower the surface tension that is an ink physical property. Accordingly, in adding the surface-active agent in a large quantity, there is a possibility of bringing about problems such that it becomes unable to eject the ink from an ink-jet recording head and that the ink permeates into the paper so excessively as to result in a low print density. That is, in the above ink-jet recording inks, the silicone type surface-active agent is only added to the ink so far as the ink can be ejected. Thus, any special effect on recorded images as aimed in the present invention is considered not to have been brought out.
In contrast thereto, as a technique in which the silicone type polymer is added to the ink, a proposal is made on achieving an improvement in water resistance of images by adding a water-insoluble silicone type vinyl polymer (see Japanese Patent Application Laid-open No. 2004-346173). In such a publication, it is disclosed to use a silicone type vinyl polymer having been made insoluble in water by copolymerizing a silicone macromer with a monomer having long-chain alkyl group having 12 or more carbon atoms or a styrene macromer. Then, it is disclosed that the use of such a polymer in water base inks for ink-jet recording brings an improvement in water resistance and storage stability of images and can impart a high print density. It is further disclosed that, the above polymer is so low hydrophilic as not to easily permeate into printing paper, so that the vinyl polymer remains on the paper surface, and hence the water resistance of recorded images can be improved, and also the coloring material can be inhibited from permeating into the printing paper to make higher the print density of the print obtained.
However, according to studies made by the present inventors, the ink containing the water-insoluble silicone type vinyl polymer as above causes a problem as stated below, especially when images are printed by the above ink-jet recording process making use of an electrothermal transducer as a means for supplying the ejection energy. That is, it has been found that, in the case of such an ink-jet recording process, physical properties of ink change abruptly because the heat energy is applied to the ink at the time of bubbling, so that, e.g., components insoluble in water may come to be deposited in nozzles to make stable ejection performance not easily achievable.